The present invention relates broadly to form-in-place (FIP) electromagnetic interference (EMI) shielding gaskets or seals, and particularly to a corrosion-resistant EMI shielding gasket, as formed and cured in place under atmospheric pressure on the surface of a substrate, which exhibits unexpectedly high levels of EMI shielding effectiveness.
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed "electromagnetic interference" or "EMI" as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other "target" devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. For further information on specifications for EMI shielding gaskets, reference may be had to Severinsen, J., "Gaskets That Block EMI," Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975).
EMI shielding gaskets typically are constructed as a resilient core element having gap-filling capabilities which is either filled, sheathed, or coated with an electrically conductive element. The resilient core element, which may be foamed or unfoamed, solid or tubular, typically is formed of an elastomeric thermoplastic material such as polyethylene, polypropylene, polyvinyl chloride, or a polypropylene-EPDM blend, or a thermoplastic or thermosetting rubber such as a butadiene, styrene-butadiene, nitrile, chlorosulfonate, neoprene, urethane, or silicone rubber.
Conductive materials for the filler, sheathing, or coating include metal or metal-plated particles, fabrics, meshes, and fibers. Preferred metals include copper, nickel, silver, aluminum, tin or an alloy such as Monel, with preferred fibers and fabrics including natural or synthetic fibers such as cotton, wool, silk, cellulose, polyester, polyamide, nylon, polyimide. Alternatively, other conductive particles and fibers such as carbon, graphite, or a conductive polymer material may be substituted.
Conventional manufacturing processes for EMI shielding gaskets include extrusion, molding, or die-cutting, with molding or die-cutting heretofore being preferred for particularly small or complex shielding configurations. In this regard, die-cutting involves the forming of the gasket from a cured sheet of an electrically-conductive elastomer which is cut or stamped using a die or the like into the desired configuration. Molding, in turn, involves the compression or injection molding of an uncured or thermoplastic elastomer into the desired configuration.
More recently, a form-in-place (FIP) process has been proposed for the manufacture of EMI shielding gaskets. As is described in commonly-assigned, co-pending applications U.S. Ser. No. 08/119,403 filed Sep. 10, 1993, abandoned; U.S. Ser. Nos. 08/375,853 filed Jan. 20, 1995, abandoned; 08/377,412 filed Jan. 24, 1995, now U.S. Pat. No. 5,641,438; U.S. Ser. No. 08/421,847 filed Mar. 14, 1995, abandoned; U.S. Ser. No. 08/422,629 filed Apr. 14, 1995, abandoned; and U.S. Ser. No. 08/761,419 filed Dec. 6, 1996, abandoned; and in European Patent EP 629,114; German Patent DE 4,319,965; Canadian Patent Application 2,125,742; and Japanese Patent Publication (Kokai) No. 7177/1993, such process involves the application of a bead of a viscous, curable, electrically-conductive composition in a fluent state to a surface of a substrate such as a housing or other enclosure. The composition, typically a silver-filled silicone elastomer, then is cured-in-place via the application of heat or with atmospheric moisture or ultraviolet (UV) radiation to form an electrically-conductive, elastomeric EMI shielding gasket in situ on the substrate surface. By forming and curing the gasket in place directly on the substrate surface, the need for separate forming and installation steps is obviated. Moreover, the gasket may be adhered directly to the surface of the substrate to further obviate the need for a separate adhesive component or other means of attachment of the gasket to the substrate. In contrast to more conventional die cutting or molding processes, the flashless FIP process reduces waste generation, and additionally is less labor intensive in that the need for hand assembly of complex gasket shapes or the mounting of the gasket into place is obviated. The process further is amenable to an automated or roboticly-controlled operation, and may be employed to fabricate complex gasket geometries under atmospheric pressure and without the use of a mold.
As the above-described FIP process continues to garner commercial acceptance, it will be appreciated that further improvements in this process and in materials therefor would be well-received by the electronics industry. In this regard, electronic devices frequently are used outdoors or otherwise are exposed to harsh service environments which may include wet, humid, or other corrosive conditions. Within such environments, the silver filler of conventional FIP gasket compositions may be subjected to galvanic or other corrosion resulting in the formation of an non-conductive surface oxide layer and, ultimately, a reduction in the EMI shielding effectiveness of the gasket. Especially desired therefore would be a corrosion-resistant, silicone-based FIP composition which could be easily and economically applied in a fluent state directly to a housing or other enclosure for forming an electrically-conductive EMI shielding gasket.